Led driver circuit and method

ABSTRACT

An apparatus includes a digital-to-analog converter coupled in series with a source follower, wherein the digital-to-analog converter is configured to control a current flowing through the source follower, and an amplifier having a first input coupled to a reference generator, a second input coupled to a common node of the source follower and the digital-to-analog converter, and an output coupled to a gate of the source follower.

TECHNICAL FIELD

The description relates generally to driver circuits, and in particularembodiments to a light-emitting diode (LED) current adjustment apparatusin a driver circuit and a corresponding control method.

BACKGROUND

A light-emitting diode (LED) is an electronic device that emits lightwhen an electric current flows through it. In order to properly drivethe LED so as to provide more efficient and reliable LED lighting, adriver circuit is employed to produce the current and voltage necessaryto turn on the LED. For example, the driver circuit is configured tocovert a dc voltage such as 12 V from a battery pack to a regulatedcurrent for driving the LED.

The most common LED driver is a voltage controlled current source. Thevoltage controlled current source is coupled in series with an LED or aplurality of LEDs. The voltage controlled current source comprises afirst current adjustment portion and a second current adjustment portionused alternatively.

The first current adjustment portion is configured to provide a coarseadjustment of the current flowing through the LED. The second currentadjustment portion is configured to provide a fine adjustment of thecurrent flowing through the LED.

The first current adjustment portion of the LED driver comprises a firstdigital-to-analog converter, a first amplifier, a first source followerand a first resistor. The first source follower and the first resistorare coupled in series. The first digital-to-analog converter isconfigured to receive a first digital reference signal, and convert thissignal into a first analog reference signal fed into a non-invertinginput of the first amplifier. An inverting input of the first amplifieris coupled to a common node of the first source follower and the firstresistor. An output of the first amplifier is coupled to a gate of thefirst source follower. The first analog reference signal is applied tothe first resistor. The current flowing through the first resistor canbe adjusted in a coarse manner through adjusting the first analogreference signal.

The second current adjustment portion of the LED driver comprises asecond digital-to-analog converter, a second amplifier, a second sourcefollower and a second resistor. The second source follower and thesecond resistor are coupled in series. The second digital-to-analogconverter is configured to receive a second digital reference signal,and convert this signal into a second analog reference signal fed into anon-inverting input of the second amplifier. An inverting input of thesecond amplifier is coupled to a common node of the second sourcefollower and the second resistor. An output of the second amplifier iscoupled to a gate of the second source follower. The second analogreference signal is applied to the second resistor. The current flowingthrough the second resistor can be adjusted in a fine manner throughadjusting the second analog reference signal.

The LED driver described above requires two digital-to-analog convertersfor programming the LED current. Furthermore, the LED driver requirestwo amplifiers and two resistors for the fine current adjustment and thecoarse current adjustment, respectively. The resistance values of thefirst resistor and the second resistor may vary when the temperaturechanges. The temperature-dependent characteristics of these tworesistors may result in deteriorating accuracy during the LED currentadjustment processes. It is desirable to have a simple and reliable LEDdriver to regulate and adjust the current flowing through the LEDaccurately.

SUMMARY

In accordance with an embodiment, an apparatus comprises adigital-to-analog converter coupled in series with a source follower,wherein the digital-to-analog converter is configured to control acurrent flowing through the source follower, and an amplifier having afirst input coupled to a reference generator, a second input coupled toa common node of the source follower and the digital-to-analogconverter, and an output coupled to a gate of the source follower.

In accordance with another embodiment, a method comprises configuring andigital-to-analog converter to operate in a fine current adjustment modeto control a current flowing through a source follower coupled in serieswith the digital-to-analog converter, and configuring thedigital-to-analog converter to operate in a coarse current adjustmentmode to control the current flowing through the source follower coupledin series with the digital-to-analog converter, wherein thedigital-to-analog converter comprises a first digital-to-analogconversion device controlled by lower bits of a digital signal, and asecond digital-to-analog conversion device controlled by upper bits ofthe digital signal.

In accordance with yet another embodiment, an apparatus comprises afirst digital-to-analog converter configured to receive lower bits of adigital signal and convert the lower bits into a first analog current, asecond digital-to-analog converter configured to receive upper bits ofthe digital signal and convert the upper bits into a second analogcurrent, a source follower having a first drain/source terminal coupledto an electronic device, and a second drain/source terminal coupled tothe first digital-to-analog converter and the second digital-to-analogconverter, wherein a current flowing through the source follower isequal to a sum of the first analog current and the second analogcurrent, and an amplifier having a first input coupled to a referencegenerator, a second input coupled to the second drain/source terminal ofthe source follower, and an output coupled to a gate of the sourcefollower.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure in order that the detaileddescription of the disclosure that follows may be better understood.Additional features and advantages of the disclosure will be describedhereinafter which form the subject of the claims of the disclosure. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present disclosure. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the disclosure as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of an LED current adjustmentapparatus in accordance with various embodiments of the presentdisclosure;

FIG. 2 illustrates a schematic diagram of the LED current adjustmentapparatus shown in FIG. 1 in accordance with various embodiments of thepresent disclosure;

FIG. 3 illustrates a schematic diagram of the reference generator shownin FIG. 1 in accordance with various embodiments of the presentdisclosure;

FIG. 4 illustrates a block diagram of the decoder operating in a finecurrent adjustment mode in accordance with various embodiments of thepresent disclosure;

FIG. 5 illustrates a block diagram of the decoder operating in a coarsecurrent adjustment mode in accordance with various embodiments of thepresent disclosure;

FIG. 6 illustrates a schematic diagram of the R2R digital-to-analogconverter shown in FIG. 2 in accordance with various embodiments of thepresent disclosure;

FIG. 7 illustrates a schematic diagram of a binary weighteddigital-to-analog converter shown in FIG. 2 in accordance with variousembodiments of the present disclosure; and

FIG. 8 illustrates a flow chart of a method for controlling the LEDcurrent adjustment apparatus shown in FIG. 1 in accordance with variousembodiments of the present disclosure.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of embodiments of this disclosure are discussed indetail below. It should be appreciated, however, that the conceptsdisclosed herein can be embodied in a wide variety of specific contexts,and that the specific embodiments discussed herein are merelyillustrative and do not serve to limit the scope of the claims. Further,it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of this disclosure as defined by the appended claims.

The present disclosure will be described with respect to preferredembodiments in a specific context, namely an LED current adjustmentapparatus. The present disclosure may also be applied, however, to avariety of systems and applications that adjust a current flowingthrough an electronic device. For example, the present disclosure may beapplied to applications where either a sink current (e.g., commoncathode LED panels) or a source current (e.g., common anode LED panels)is required. Hereinafter, various embodiments will be explained indetail with reference to the accompanying drawings.

FIG. 1 illustrates a block diagram of an LED current adjustmentapparatus in accordance with various embodiments of the presentdisclosure. The LED current adjustment apparatus wo comprises a driver102, a digital-to-analog converter 104, a reference generator 112 and adecoder 114. As shown in FIG. 1, the driver 102 and thedigital-to-analog converter 104 are coupled in series between an LED (alight emitting diode or a plurality of light emitting diodes) andground. The reference generator 112 is configured to provide a referencesignal fed into the driver 102. In some embodiments, the referencesignal is a voltage reference signal applied to the digital-to-analogconverter 104 through the driver 102. The decoder 114 is configured toreceive a system LED current adjustment command, and convert this systemLED current adjustment command into a digital signal, which is appliedto the digital-to-analog converter 104. In response to this digitalsignal, the digital-to-analog converter 104 is capable of adjusting thecurrent flowing through the LED.

In some embodiments, the digital-to-analog converter 104 is implementedas a segmented digital-to-analog converter. The segmenteddigital-to-analog converter may be configured as a coarse and/or finedigital-to-analog converter. Such a configuration can be used to replacetwo digital-to-analog converters (two DACs for coarse and fine currentadjustment respectively) commonly used in a conventional LED currentadjustment apparatus. Throughout the description, the digital-to-analogconverter 104 may be alternatively referred to as a segmenteddigital-to-analog converter.

The segmented digital-to-analog converter 104 comprises a firstdigital-to-analog conversion device and a second digital-to-analogconversion device. In some embodiments, the first digital-to-analogconversion device is controlled by lower bits of the digital signal.More particularly, the first digital-to-analog conversion deviceconverts the lower bits of the digital signal into a first analogcurrent. The second digital-to-analog conversion device is controlled byupper bits of the digital signal. More particularly, the seconddigital-to-analog conversion device converts the upper bits of thedigital signal into a second analog current. The current flowing throughthe driver 102 is the sum of the first analog current and the secondanalog current.

In some embodiments, the first digital-to-analog conversion device is aR2R digital-to-analog converter. The R2R digital-to-analog converter isalso known as an R-2R resistor ladder network. The seconddigital-to-analog conversion device is a thermometric digital-to-analogconverter. In some embodiments, the R2R digital-to-analog converter iscontrolled by L bits of the digital signal. L is an integer equal orgreater than one. The L bits are the lower bits of the digital signal.The thermometric digital-to-analog converter is controlled by M bits ofthe digital signal. M is an integer equal or greater than one. The Mbits are the upper bits of the digital signal.

In operation, the LED current adjustment apparatus wo may be configuredto operate in different current adjustment modes. In some embodiments,the LED current adjustment apparatus wo is configured to operate in afine current adjustment mode. In the fine current adjustment mode, thedigital signal has 15 bits. L is equal to 6, and M is equal to 9. Insome embodiments, the LED current adjustment apparatus wo is configuredto operate in a coarse current adjustment mode. In the coarse currentadjustment mode, the digital signal has 10 bits. The four mostsignificant bits of this digital signal are tied to zero. The leastsignificant bits are shifted to the left by one bit. L is equal to 5,and M is equal to 9 (four of the most significant bits are tied tozero). The detailed structures and operating principles of the R2Rdigital-to-analog converter and the thermometric digital-to-analogconverter will be described below with respect to FIGS. 4-7.

In operation, the R2R digital-to-analog converter and the thermometricdigital-to-analog converter are configured to leave the fine currentadjustment mode and enter into the coarse current adjustment mode.During a transition from the fine current adjustment mode to the coarsecurrent adjustment mode, control bits for controlling the R2Rdigital-to-analog converter are shifted to the left by one bit.

In operation, the R2R digital-to-analog converter and the thermometricdigital-to-analog converter are configured to operate in the finecurrent adjustment mode. In the fine current adjustment mode, the R2Rdigital-to-analog converter and is configured as a 6-bit R2Rdigital-to-analog converter. The thermometric digital-to-analogconverter is configured as a 9-bit thermometric digital-to-analogconverter.

In operation, the R2R digital-to-analog converter and the thermometricdigital-to-analog converter are configured to operate in the coarsecurrent adjustment mode. In the coarse current adjustment mode, the R2Rdigital-to-analog converter is configured as a 6-bit R2Rdigital-to-analog converter. The least significant bit of the 6-bit R2Rdigital-to-analog converter is tied to a logic high state. Thethermometric digital-to-analog converter is configured as a 9-bitthermometric digital-to-analog converter. The four most significant bitsof the 9-bit thermometric digital-to-analog converter are tied to alogic low state.

FIG. 2 illustrates a schematic diagram of the LED current adjustmentapparatus shown in FIG. 1 in accordance with various embodiments of thepresent disclosure. The driver 102 comprises a first amplifier 152 and asource follower S1. As shown in FIG. 2, the source follower S1 isimplemented as an n-type transistor. A first drain/source terminal ofthe source follower S1 is a drain of the source follower. The drain ofthe source follower S1 is coupled to the LED. In other words, thecurrent flowing through the source follower S1 is substantially equal tothe current flowing through the LED. A second drain/source terminal ofthe source follower S1 is a source of the source follower. The source ofthe source follower S1 is coupled to the segmented digital-to-analogconverter 104. The gate of the source follower S1 is coupled to theoutput of the first amplifier 152. The non-inverting input of the firstamplifier 152 is coupled to a first voltage bus V1. The inverting inputof the first amplifier 152 is coupled to a second voltage bus V2. Thesecond voltage bus V2 is also coupled to the source of the sourcefollower S1.

The reference generator 112 comprises a second amplifier 154. Thenon-inverting input of the second amplifier 154 is configured to receivea reference signal VREF. The reference signal VREF is generated by acurrent source and a resistor, which will be described in detail belowwith respect to FIG. 3. The inverting input of the second amplifier 154is coupled to the output of the second amplifier 154. The output of thesecond amplifier 154 is coupled to the non-inverting input of the firstamplifier 152. As shown in FIG. 2, the second amplifier 154 isconfigured as a buffer. The output voltage of the second amplifier 154is substantially equal to the voltage of the reference signal VREF.

It should be noted that both the first amplifier 152 and the secondamplifier 154 may have an offset. In some embodiments, this offset isabout +/−10 millivolts at a five-sigma level. Furthermore, the resistormay have a mismatch (<0.5%). Both the mismatch of the resistor and theoffset of the amplifiers can be compensated by a suitable currenttrimming circuit.

The segmented digital-to-analog converter 104 is formed by twodigital-to-analog converters, namely a R2R digital-to-analog converter122 and a thermometric digital-to-analog converter 124. The detailedschematic diagrams of these two digital-to-analog converters will bedescribed below with respect to FIGS. 6 and 7, respectively.

As shown in FIG. 2, the R2R digital-to-analog converter 122 isconfigured to receive a digital signal generated by the decoder 114.More particularly, lower bits of the digital signal are fed into the R2Rdigital-to-analog converter 122. The R2R digital-to-analog converter 122is also coupled to the voltage buses V1 and V2 as shown in FIG. 2. Inresponse to the received digital signal (lower bits of the digitalsignal), the R2R digital-to-analog converter 122 converts the lower bitsof the digital signal into an analog current flowing through the R2Rdigital-to-analog converter 122. Since the inputs of the first amplifier152 are high-impedance inputs, the current flowing through the R2Rdigital-to-analog converter 122 comes from the current flowing throughthe source follower S1. The detailed operating principle of the R2Rdigital-to-analog converter 122 will be described below with respect toFIG. 6.

As shown in FIG. 2, the thermometric digital-to-analog converter 124 isconfigured to receive the digital signal generated by the decoder 114.More particularly, upper bits of the digital signal are fed into thethermometric digital-to-analog converter 124. The thermometricdigital-to-analog converter 124 is also coupled to the voltage bus V2 asshown in FIG. 2. In response to the received digital signal (upper bitsof the digital signal), the thermometric digital-to-analog converter 124converts the upper bits of the digital signal into an analog currentflowing through the thermometric digital-to-analog converter 124. Sincethe inverting input of the first amplifier 152 is a high-impedanceinput, the current flowing through the thermometric digital-to-analogconverter 124 comes from the current flowing through the source followerS1. The detailed operating principle of the thermometricdigital-to-analog converter 124 will be described below with respect toFIG. 7.

It should be noted the current flowing through the source follower S1 isthe sum of the current flowing through the R2R digital-to-analogconverter 122 and the current flowing through the thermometricdigital-to-analog converter 124. By generating different digitalsignals, the currents flowing through the R2R digital-to-analogconverter 122 and the thermometric digital-to-analog converter 124 mayvary accordingly. As a result, the current flowing through the sourcefollower S1 may change in response to the variation of the digitalsignal. As described above, the current flowing through the sourcefollower S1 is substantially equal to the current flowing through theLED. As such, the current of the LED can be controlled through adjustingthe output of the decoder 114.

In operation, the first amplifier 152 having an output coupled to thegate of the source follower S1 forces the voltage of the source of thesource follower S1 to be at about V1 (VREF) by forcing the differencebetween the inverting and non-inverting terminals of the first amplifier152 to be about zero volts. As such, the voltage applied to the R2Rdigital-to-analog converter 122 and the thermometric digital-to-analogconverter 124 is substantially equal to VREF.

One advantageous feature of the LED current adjustment apparatus shownin FIG. 2 is that a single digital-to-analog converter (e.g., segmenteddigital-to-analog converter 104) is employed to achieve two differentcurrent adjustments. This simple solution helps to reduce temperatureand process related variations. Furthermore, this solution helps toreduce the silicon area consumption, thereby reducing the cost of theLED current adjustment apparatus.

FIG. 3 illustrates a schematic diagram of the reference generator shownin FIG. 1 in accordance with various embodiments of the presentdisclosure. The reference generator 112 comprises a current source IBG,a resistor R_(REF) and the second amplifier 154. As shown in FIG. 3, thecurrent source IBG and the resistor R_(REF) are coupled in seriesbetween a bias power supply VDD and ground. The common node of thecurrent source IBG and the resistor R_(REF) is coupled to thenon-inverting input of the second amplifier 154. The inverting input ofthe second amplifier 154 is coupled to the output of the secondamplifier 154.

In operation, the current of the current source IBG flows through theresistor R_(REF) to generate a digital-to-analog conversion voltagereference VREF. The current of the current source IBG is a temperaturecompensated current. In some embodiments, the temperature compensatedcurrent of the current source IBG may be achieved through suitabletemperature compensation techniques such as the temperature compensationtechnique used in the bandgap voltage reference.

The temperature compensated current of the current source IBG has thesmallest temperature coefficient (TC). For example, the TC of thecurrent of the current source IBG is less than 30 ppm/degree. In someembodiments, the TC of the current of the current source IBG is aboutzero.

The resistor R_(REF) and the resistors in the segmenteddigital-to-analog converter 104 (shown in FIGS. 6 and 7) are the samekind of resistors. In some embodiments, the temperature and/or processdrifts of the resistor R_(REF) and the resistors in the segmenteddigital-to-analog converter 104 are equal and opposite. As a result, theimpacts from the resistor R_(REF) and the resistors in the segmenteddigital-to-analog converter 104 may cancel each other out.

One advantageous feature of having a temperature compensated referencegenerator is by temperature compensating the reference generator, anaccurate voltage reference can be ensured over a wide range oftemperatures. Accordingly, the current flowing through the LED can beensured to be sufficiently accurate over various operating conditions.

FIG. 4 illustrates a block diagram of the decoder operating in a finecurrent adjustment mode in accordance with various embodiments of thepresent disclosure. In some embodiments, the LED current adjustmentapparatus 100 is configured to operate two different operating modes,namely a fine current adjustment mode and a coarse current adjustmentmode. In response to the fine current adjustment mode, the decoder 114is configured to generate a digital signal having 15 bits. The lower sixbits (b0-b5) are fed into the R2R digital-to-analog converter 122. Inthe R2R digital-to-analog converter 122, the lower five bits areconverted into a corresponding analog current. The upper nine bits(b6-b14) are fed into the thermometric digital-to-analog converter 124.In the thermometric digital-to-analog converter 124, the upper nine bitsare converted into a corresponding analog current. The sum of thecurrents flowing through the R2R digital-to-analog converter 122 and thethermometric digital-to-analog converter 124 is the current flowingthrough the LED in the fine current adjustment mode.

FIG. 5 illustrates a block diagram of the decoder operating in a coarsecurrent adjustment mode in accordance with various embodiments of thepresent disclosure. In response to the coarse current adjustment mode,the decoder 114 is configured to generate a digital signal having 15bits. During the transition from the fine current adjustment mode to thecoarse current adjustment mode, the lower bits of the digital signal areshifted to the left by one bit, and four most significant bits of thedigital signal are tied to a logic high state. As such, the lower fivebits (1, b0-b4) are fed into the R2R digital-to-analog converter 122. Inthe R2R digital-to-analog converter 122, the lower five bits areconverted into a corresponding analog current. The upper five bits(b5-b9) are fed into the thermometric digital-to-analog converter 124.In the thermometric digital-to-analog converter 124, the upper five bitsare converted into a corresponding analog current. The sum of thecurrents flowing through the R2R digital-to-analog converter 122 and thethermometric digital-to-analog converter 124 is the current flowingthrough the LED in the coarse current adjustment mode.

FIG. 6 illustrates a schematic diagram of the R2R digital-to-analogconverter shown in FIG. 2 in accordance with various embodiments of thepresent disclosure. In some embodiments, the R2R digital-to-analogconverter may have n number of branches. In some embodiments, n is aninteger equal or greater than one. In some embodiments, the R2Rdigital-to-analog converter 122 is implemented as a R2Rdigital-to-analog converter having six branches as shown in FIG. 6.

As shown in FIG. 6, each branch 602 includes a horizontal resistor Rhaving a resistor value “R” and a vertical resistor (two resistorscoupled in series) having a resistor value “2R” as shown in FIG. 6. Thevertical resistor of each branch is coupled to the voltage buses V1 andV2 through a control switch (e.g., control switch S0-S5). One bit of thedigital signal is used to control a corresponding control switch.Depending on the logic state of the bit of the digital signal, thevertical resistor may be coupled to either the first voltage bus V1 orthe second voltage bus V2. The R2R digital-to-analog converter furthercomprises a dummy branch (two rightmost resistors). As shown in FIG. 6,the dummy branch is coupled to the second voltage bus V2.

In operation, the R2R digital-to-analog converter 122 may be used inconverting a digital signal into a corresponding analog signal. Forexample, the R2R digital-to-analog converter 122 may generate a currentcorresponding to the digital signal. The operating principle of the R2Rdigital-to-analog converter is well known in the art, and hence is notdiscussed in further detail to avoid repetition.

It should be noted that the diagram shown in FIG. 6 is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, one skilled in the art would recognizethat the R2R digital-to-analog converter shown in FIG. 6 is simply onemanner of generating the LED current and that other and alternateembodiment digital-to-analog converters could be employed (such asemploying a binary weighted converter) for generating the LED current.The final choice is dictated by the resolution and precision required inthe application.

Referring back to FIG. 1, the LED current adjustment apparatus wo may beconfigured to operate in the fine current adjustment mode. In the finecurrent adjustment mode, the current flowing through the R2Rdigital-to-analog converter can be expressed by the following equation:

$\begin{matrix}{I_{R2R\_{fine}} = {{IBG} \times \frac{R_{REF}}{R} \times \left( {\frac{b\; 5}{2^{1}} + \frac{b\; 4}{2^{2}} + \frac{b\; 3}{2^{3}} + \frac{b\; 2}{2^{4}} + \frac{b\; 1}{2^{5}} + \frac{b\; 0}{2^{6}}} \right)}} & (1)\end{matrix}$

Alternatively, the LED current adjustment apparatus wo may be configuredto operate in the coarse current adjustment mode. In the coarse currentadjustment mode, the current flowing through the R2R digital-to-analogconverter can be expressed by the following equation:

$\begin{matrix}{I_{R2R\_{coarse}} = {{IBG} \times \frac{R_{REF}}{R} \times \left( {\frac{b\; 4}{2^{1}} + \frac{b\; 3}{2^{2}} + \frac{b\; 2}{2^{3}} + \frac{b\; 1}{2^{4}} + \frac{b\; 0}{2^{5}} + \frac{1}{2^{6}}} \right)}} & (2)\end{matrix}$

FIG. 7 illustrates a schematic diagram of a binary weighteddigital-to-analog converter shown in FIG. 2 in accordance with variousembodiments of the present disclosure. In some embodiments, thethermometric digital-to-analog converter may have n number of branches.In some embodiments, n is an integer equal or greater than one. In someembodiments, the thermometric digital-to-analog converter 124 isimplemented as a binary weighted digital-to-analog converter having ninebranches 701-709 as shown in FIG. 7.

As shown in FIG. 7, each branch is a binary weighted current source. Inthe first branch 701, the current of this branch is equal to V2 dividedby R. In the second branch 702, two resistors are connected in parallel.The equivalent resistance is one half of that of the first branch 701.As such, the current of the second branch is 702 is twice as much asthat of the first branch 701. In the third branch 703, four resistorsare connected in parallel. The current of the third branch is 703 istwice as much as that of the second branch 702. In sum, for two adjacentbranches (e.g., branches 707 and 708), the current of the right branch(e.g., branch 708) is twice as much as that of the left branch (e.g.,branch 707).

In operation, the thermometric digital-to-analog converter 124 may beused in converting a digital signal into a corresponding analog signal.For example, the thermometric digital-to-analog converter 124 maygenerate a current corresponding to the digital signal. The operatingprinciple of the thermometric digital-to-analog converter is well knownin the art, and hence is not discussed in further detail to avoidrepetition.

It should be noted that the diagram shown in FIG. 7 is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, one skilled in the art would recognizethat the thermometric digital-to-analog converter shown in FIG. 7 issimply one manner of generating the LED current and that other andalternate embodiment digital-to-analog converters could be employed forthis function. The final choice is dictated by the resolution andprecision required in the application.

Referring back to FIG. 1, the LED current adjustment apparatus wo may beconfigured to operate in the fine current adjustment mode. In the finecurrent adjustment mode, the current flowing through the thermometricdigital-to-analog converter 124 can be expressed by the followingequation:

$\begin{matrix}{I_{{TM}\_{fine}} = {{IBG} \times \frac{R_{REF}}{R} \times \left( {{b\;{14 \cdot 2^{8}}} + {b\;{13 \cdot 2^{7}}} + {b\;{12 \cdot 2^{6}}} + {b\;{11 \cdot 2^{5}}} + {b\;{10 \cdot 2^{4}}} + {b\;{9 \cdot 2^{3}}} + {b\;{8 \cdot 2^{2}}} + {b\;{7 \cdot 2^{1}}} + {b\;{6 \cdot 2^{0}}}} \right)}} & (3)\end{matrix}$

Alternatively, the LED current adjustment apparatus wo may be configuredto operate in the coarse current adjustment mode. In the coarse currentadjustment mode, the current flowing through the thermometricdigital-to-analog converter 124 can be expressed by the followingequation:

$\begin{matrix}{I_{R2R\_{coarse}} = {{IBG} \times \frac{R_{REF}}{R} \times \left( {{0 \cdot 2^{8}} + {0 \cdot 2^{7}} + {0 \cdot 2^{6}} + {0 \cdot 2^{5}} + {b\;{9 \cdot 2^{4}}} + {b\;{8 \cdot 2^{3}}} + {b\;{7 \cdot 2^{2}}} + {b\;{6 \cdot 2^{1}}} + {b\;{5 \cdot 2^{0}}}} \right)}} & (4)\end{matrix}$

Referring back to FIG. 2, the current flowing through the LED is the sumof the current flowing through the thermometric digital-to-analogconverter 124 and the current flowing through R2R digital-to-analogconverter 122. The current flowing through the LED can be expressed bythe following equation:

$\begin{matrix}{I_{LED} = {{IBG} \times \frac{R_{REF}}{R} \times \left( {{\sum\limits_{m = 0}^{M - 1}\;{b_{m + L} \cdot 2^{m}}} + {\sum\limits_{n = 0}^{L - 1}\;\frac{bn}{2^{L - n}}} + \frac{1}{2^{L}}} \right)}} & (5)\end{matrix}$

In Equation (5), in some embodiments, M is equal to 9, and L is equal to6. The last element (½^(L)) is an offset. This offset is employed to setthe full scale value at 2^(L+M)×IBG×R_(REF)/R.

FIG. 8 illustrates a flow chart of a method for controlling the LEDcurrent adjustment apparatus shown in FIG. 1 in accordance with variousembodiments of the present disclosure. This flowchart shown in FIG. 8 ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. For example, various stepsillustrated in FIG. 8 may be added, removed, replaced, rearranged andrepeated.

An LED current adjustment apparatus comprises a digital-to-analogconverter. The digital-to-analog converter is formed by a firstdigital-to-analog converter and a second digital-to-analog converter.The first digital-to-analog converter is configured to receive lowerbits of a digital signal, and convert the lower bits into a first analogcurrent. The second digital-to-analog converter is configured to receiveupper bits of the digital signal, and convert the upper bits into asecond analog current. The first digital-to-analog converter is a R2Rdigital-to-analog converter. The second digital-to-analog converter is athermometric digital-to-analog converter.

The LED current adjustment apparatus further comprises a source followerand an amplifier. The source follower has a first drain/source terminalcoupled to an electronic device (e.g., an LED), and a seconddrain/source terminal coupled to the digital-to-analog converter. Acurrent flowing through the source follower is equal to a sum of thefirst analog current and the second analog current. The amplifier has afirst input coupled to a reference generator, a second input coupled tothe second drain/source terminal of the source follower, and an outputcoupled to a gate of the source follower.

The LED current adjustment apparatus may be configured to operate indifferent current adjustment modes, namely a fine current adjustmentmode and a coarse current adjustment mode.

At step 802, the digital-to-analog converter is configured to operate inthe fine current adjustment mode to control the current flowing throughthe source follower coupled in series with the digital-to-analogconverter. In the fine current adjustment mode, the firstdigital-to-analog conversion device is configured as a 6-bit R2Rdigital-to-analog converter, and the second digital-to-analog conversiondevice is configured as a 9-bit thermometric digital-to-analogconverter.

At step 804, the digital-to-analog converter is configured to operate inthe coarse current adjustment mode to control the current flowingthrough the source follower coupled in series with the digital-to-analogconverter. In the coarse current adjustment mode, the firstdigital-to-analog conversion device is configured as a 6-bit R2Rdigital-to-analog converter having the least significant bit tied to alogic high state. The second digital-to-analog conversion device isconfigured as a 9-bit thermometric digital-to-analog converter havingfour most significant bits tied to a logic low state.

It should be noted that the flow chart shown in FIG. 8 is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, depending on different applications anddesign needs, the coarse current adjustment mode may be executed priorto the fine current adjustment mode.

Although embodiments of the present disclosure and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present disclosure, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present disclosure. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. An apparatus comprising: a digital-to-analog converter coupled inseries with a source follower, wherein the digital-to-analog converteris a segmented digital-to-analog converter, and the digital-to-analogconverter is configured to control a current flowing through the sourcefollower; and an amplifier having a first input coupled to a referencegenerator, a second input coupled to a common node of the sourcefollower and the digital-to-analog converter, and an output coupled to agate of the source follower.
 2. The apparatus of claim 1, wherein thedigital-to-analog converter comprises: a first digital-to-analogconversion device controlled by lower bits of a digital signal; and asecond digital-to-analog conversion device controlled by upper bits ofthe digital signal.
 3. The apparatus of claim 2, further comprising: thefirst digital-to-analog conversion device is a R2R digital-to-analogconverter; and the second digital-to-analog conversion device is athermometric digital-to-analog converter.
 4. The apparatus of claim 3,wherein: the R2R digital-to-analog converter and the thermometricdigital-to-analog converter are configured to operate in a fine currentadjustment mode, and wherein under the fine current adjustment mode, thefirst digital-to-analog conversion device is configured as a 6-bit R2Rdigital-to-analog converter, and the second digital-to-analog conversiondevice is configured as a 9-bit thermometric digital-to-analogconverter.
 5. The apparatus of claim 3, wherein: the R2Rdigital-to-analog converter and the thermometric digital-to-analogconverter are configured to operate in a coarse current adjustment mode,and wherein under the coarse current adjustment mode, the firstdigital-to-analog conversion device is configured as a 6-bit R2Rdigital-to-analog converter having a least significant bit tied to alogic high state, and the second digital-to-analog conversion device isconfigured as a 9-bit thermometric digital-to-analog converter havingfour most significant bits tied to a logic low state.
 6. The apparatusof claim 3, wherein: the R2R digital-to-analog converter and thethermometric digital-to-analog converter are configured to leave a finecurrent adjustment mode and enter into a coarse current adjustment mode,and wherein during a transition from the fine current adjustment mode tothe coarse current adjustment mode, control bits for controlling the R2Rdigital-to-analog converter are shifted to the left by one bit.
 7. Theapparatus of claim 1, wherein: the reference generator comprises acurrent source and a reference resistor and a reference amplifier,wherein: the current source and the reference resistor are coupled inseries; a first input of the reference amplifier is coupled to a commonnode of the current source and the reference resistor; and a secondinput of the reference amplifier is coupled to an output of thereference amplifier, and wherein the output of the reference amplifieris coupled to the first input of the amplifier.
 8. The apparatus ofclaim 1, further comprising: a decoder configured to generate a digitalsignal fed into the digital-to-analog converter.
 9. (canceled)
 10. Amethod comprising: configuring an digital-to-analog converter to operatein a fine current adjustment mode to control a current flowing through asource follower coupled in series with the digital-to-analog converter;and configuring the digital-to-analog converter to operate in a coarsecurrent adjustment mode to control the current flowing through thesource follower coupled in series with the digital-to-analog converter,wherein the digital-to-analog converter comprises a firstdigital-to-analog conversion device controlled by lower bits of adigital signal, and a second digital-to-analog conversion devicecontrolled by upper bits of the digital signal.
 11. The method of claim10, further comprising: generating the digital signal using a decoder;applying the lower bits of the digital signal to the firstdigital-to-analog conversion device; and applying the upper bits of thedigital signal to the second digital-to-analog conversion device. 12.The method of claim 11, wherein: the first digital-to-analog conversiondevice is a R2R digital-to-analog converter; and the seconddigital-to-analog conversion device is a thermometric digital-to-analogconverter.
 13. The method of claim 12, further comprising: shifting thelower bits to the left by one bit in response to a mode transition fromthe fine current adjustment mode to the coarse current adjustment mode.14. The method of claim 12, further comprising: tying at least one bitof the upper bits to a logic low stage in response to a mode transitionfrom the fine current adjustment mode to the coarse current adjustmentmode.
 15. The method of claim 10, further comprising: providing anamplifier to control a gate voltage of the source follower, wherein: afirst input of the amplifier is configured to be coupled to apredetermined reference; a second input of amplifier is coupled to acommon node of the source follower and the digital-to-analog converter;and an output of the amplifier is coupled to a gate of the sourcefollower.
 16. The method of claim 15, wherein: the first input of theamplifier is a non-inverting input of the amplifier; and the secondinput of the amplifier is an inverting input of the amplifier.
 17. Anapparatus comprising: a first digital-to-analog converter configured toreceive lower bits of a digital signal and convert the lower bits into afirst analog current; a second digital-to-analog converter configured toreceive upper bits of the digital signal and convert the upper bits intoa second analog current; a source follower having a first drain/sourceterminal coupled to an electronic device, and a second drain/sourceterminal coupled to the first digital-to-analog converter and the seconddigital-to-analog converter, wherein a current flowing through thesource follower is equal to a sum of the first analog current and thesecond analog current; and an amplifier having a first input coupled toa reference generator, a second input coupled to the second drain/sourceterminal of the source follower, and an output coupled to a gate of thesource follower.
 18. The apparatus of claim 17, wherein: the firstdigital-to-analog converter is a R2R digital-to-analog converter; thesecond digital-to-analog converter is a thermometric digital-to-analogconverter; and the electronic device is a plurality of light emittingdiodes.
 19. The apparatus of claim 17, wherein: the firstdigital-to-analog converter and the second digital-to-analog converterare configured to operate in either a fine current adjustment mode or acoarse current adjustment mode, and wherein during a transition from thefine current adjustment mode to the coarse current adjustment mode,control bits for controlling the first digital-to-analog converter areshifted to the left by one bit.
 20. The apparatus of claim 17, wherein:the source follower is an n-type transistor, and wherein the firstdrain/source terminal is a drain of the source follower, and the seconddrain/source terminal is a source of the source follower.
 21. Theapparatus of claim 17, further comprising: a decoder configured togenerate the digital signal.